Serological evidence of the circulation of the Rift Valley fever virus in sheep and goats slaughtered in Yaoundé, Cameroon

Abstract Background Rift Valley fever (RVF) is an emerging mosquito‐borne haemorrhagic fever disease capable of causing severe outbreaks with high mortality and morbidity in human, livestock, and wildlife species, particularly in Africa. The onset of the disease in humans is often preceded by epizootic circulation in animals. Therefore, this study was conducted to investigate the seroprevalence of Rift Valley fever virus (RVFV) infection in animals slaughtered in the “Marché huitième” slaughterhouse in Yaoundé, Cameroon. Methods A cross‐sectional study was conducted at the “Marché huitième” slaughterhouse in Yaoundé, Centre region of Cameroon in March 2020. Blood samples of two species of small ruminants (sheep and goat) were collected and processed. Serum was analysed for detection of RVFV IgG and IgM using commercial ELISA tests. Results Of the 191 ruminants tested, RVFV IgG antibodies were positive in 10 (5.2%). Regarding categorization of the population based on the species and gender, sheep and female animal had the highest seroprevalence of 6.4% (3/47) and 7.0% (8/115), respectively. All sera from IgG antibodies‐positive samples were negative to IgM antibodies. Conclusion This study provides evidence of the circulation of RVFV in small ruminants sold and slaughtered at the “Marché huitième” slaughterhouse in Yaoundé and highlights the need to develop a surveillance system for this virus encompassing humans, livestock, wildlife, and vectors in Cameroon.

. RVF is caused by Rift Valley fever virus (RVFV), which is an arbovirus in the Phlebovirus genus and Phenuiviridae family (Adams et al., 2017;King et al., 2018). The disease is transmitted by infected mosquitoes of several genera, mainly Aedes spp. and Culex spp., and by direct contact with body fluids, blood, or tissues of viremic animals (Balenghien et al., 2013;Bird et al., 2009;Pepin et al., 2010).
RVFV infections in humans usually lead to subclinical infection or cause moderate to severe non-fatal, acute illness but some patients can develop haemorrhagic syndrome and/or encephalitis (Pepin et al., 2010) with an overall case fatality rate estimated to be between 0.5% and 2% (Madani et al., 2003;Pepin et al., 2010). In animals, infections are characterized by abortion storms, especially among pregnant domestic ruminants like goats and sheep and high mortality among newborns of domestic ruminants (Bird et al., 2009).
An increase in international domestic ruminants trade and some other factors encountered in Sub-Saharan Africa such as favourable climate, presence of a wide variety of vectors, and human behaviour may lead to RVFV outbreak (Baba et al., 2016), which can have considerable socio-economic and public health impacts (Baba et al., 2016;Peyre et al., 2015). The onset of the disease in humans is preceded generally by epizootic circulation of the virus in livestock. Surveillance of RVF by assessing past and present RVFV antibodies status in animals and humans may be a good indicator providing evidence of the circulation of the virus in a given area and helps to detect at-risk populations, to anticipate a potential outbreak.
In Cameroon, there is not yet any official report of RVF outbreaks but the circulation of the virus has been confirmed by reports of detection of antibodies against the virus in animals and humans (Gonzalez et al., 1989;LeBreton et al., 2006;Poueme et al., 2019;Rissmann et al., 2017;Sadeuh-Mba et al., 2018;Zeller et al., 1995). Of these previous studies on RVFV carried out in Cameroon on animals, only two studies were carried out in the city of Yaoundé and the samples of small ruminants collected were less than 15 (LeBreton et al., 2006;Rissmann et al., 2017). This silent circulation of RVFV shows that Cameroon may be a country at risk of RVF epidemics and indicates the need of continuous surveillance of this virus in both animals and humans. This study investigates the seroprevalence of RVFV infection in animals slaughtered in "Marché huitième" slaughterhouse, the main slaughter market for small ruminants in Yaoundé, Cameroon.

Study area
This study was carried out at the "Marché huitième" slaughterhouse in Yaoundé, Mfoundi division, Centre region of Cameroon (Figure 1) between 4 and 20 March 2020, at the end of dry season. This market is the largest for the sale and slaughter of small ruminants in Yaoundé.
An average of 150 domestic ruminants are slaughtered per day in this slaughterhouse for human consumption in private homes or delivered to supermarkets in the city (the city of Yaoundé has over 2.766 million inhabitants) and its surroundings for sale. The small ruminants slaughtered in this slaughterhouse are from various sources, mainly from the northern part of the country.

Blood sampling and laboratory analysis
A blood sample (5 ml instructions for specific detection of IgM antibodies. All samples with a percentage of inhibition higher than 50% were considered to be positive and those lower than 40% as negative. Sample with a percentage of inhibition between 40% and 50% were identified as inconclusive. All samples that tested positive for total RVFV antibodies but negative for IgM were considered as anti-RVF IgG antibodies positive.

Data analysis
The data were entered into a Microsoft Excel spreadsheet and imported into Statistical Package for Social Sciences (SPSS) software Version 23.0. Seroprevalence for RVFV was analysed by species and sex, presented as proportions, and compared using Fisher's exact test.
The differences were considered significant where the P-values were lower than 0.05.
Two inconclusive sera were observed and were from female livestock (Table 1).

DISCUSSION
Here, we report the presence of IgG antibodies against RVFV in sheep and goats slaughtered at the "Marché huitième" slaughterhouse of Yaoundé. The overall seroprevalence of RVFV IgG antibodies, which is indicative of past exposure, was 5.2%, and none of these small ruminants showed evidence of new or recent infection. The presence of IgG antibodies against RVFV among small ruminants provides evidence of the silent circulation of RVFV in Cameroon and indicates that humans, particularly breeders, workers in slaughterhouses, and livestock markets are exposed to RVFV. This silent circulation in animals, reported by previous studies in Cameroon, confirms previous findings (Gonzalez et al., 1989;LeBreton et al., 2006;Poueme et al., 2019;Rissmann et al., 2017;Sadeuh-Mba et al., 2018;Zeller et al., 1995).
The prevalence of IgG antibodies against RVFV for small ruminants found in this study is slightly higher compared to those found in other recent studies in Cameroon (Poueme et al., 2019;Rissmann et al., 2017). However, when we consider climatic/seasonal conditions (dry season) and locality (Centre region), the results of this study are similar to those obtained in previous studies (Poueme et al., 2019;Rissmann et al., 2017), suggesting the influence of climatic/seasonal and agroecological conditions in the variation of seroprevalence.
Cameroonian borders in the northern part of country are sufficiently porous and unrestricted for the movement of humans and animals and some animals sold and slaughtered at the "Marché huitième" are likely to come from these neighbouring countries. Serological and molecular studies carried out in neighbouring countries that share common borders showed a variable seroprevalence of prior and current/recent infections (Nakouné et al., 2016;Opayele et al., 2019;Ringot et al., 2004). These findings indicate possible transboundary silent circulation of RVFV between Cameroon and its neighbouring countries such as Nigeria, Chad, and the Central African Republic.